As we migrate the AB8500 GPADC driver to use IIO, we need to augment
the bindings to account for defining the ADC channels in the device
tree.

Cc: devicetree@vger.kernel.org
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>